US8733129B2 - Glass substrate for magnetic disk and manufacturing method thereof - Google Patents

Glass substrate for magnetic disk and manufacturing method thereof Download PDF

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Publication number
US8733129B2
US8733129B2 US12/979,176 US97917610A US8733129B2 US 8733129 B2 US8733129 B2 US 8733129B2 US 97917610 A US97917610 A US 97917610A US 8733129 B2 US8733129 B2 US 8733129B2
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United States
Prior art keywords
glass material
polishing
sheet glass
magnetic disk
substrate
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US12/979,176
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US20110159321A1 (en
Inventor
Shinji Eda
Hideki Isono
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Hoya Corp
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Hoya Corp
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Publication of US20110159321A1 publication Critical patent/US20110159321A1/en
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    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
    • G11B5/84—Processes or apparatus specially adapted for manufacturing record carriers
    • G11B5/8404—Processes or apparatus specially adapted for manufacturing record carriers manufacturing base layers
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
    • G11B5/62—Record carriers characterised by the selection of the material
    • G11B5/73—Base layers, i.e. all non-magnetic layers lying under a lowermost magnetic recording layer, e.g. including any non-magnetic layer in between a first magnetic recording layer and either an underlying substrate or a soft magnetic underlayer
    • G11B5/739—Magnetic recording media substrates
    • G11B5/73911—Inorganic substrates
    • G11B5/73921—Glass or ceramic substrates
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B41/00—Component parts such as frames, beds, carriages, headstocks
    • B24B41/06—Work supports, e.g. adjustable steadies
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B11/00—Pressing molten glass or performed glass reheated to equivalent low viscosity without blowing
    • C03B11/05—Pressing molten glass or performed glass reheated to equivalent low viscosity without blowing in machines with reciprocating moulds
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B11/00—Pressing molten glass or performed glass reheated to equivalent low viscosity without blowing
    • C03B11/06—Construction of plunger or mould
    • C03B11/08—Construction of plunger or mould for making solid articles, e.g. lenses
    • C03B11/088—Flat discs
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B11/00—Pressing molten glass or performed glass reheated to equivalent low viscosity without blowing
    • C03B11/005—Pressing under special atmospheres, e.g. inert, reactive, vacuum, clean
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B11/00—Pressing molten glass or performed glass reheated to equivalent low viscosity without blowing
    • C03B11/06—Construction of plunger or mould
    • C03B11/08—Construction of plunger or mould for making solid articles, e.g. lenses
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B11/00—Pressing molten glass or performed glass reheated to equivalent low viscosity without blowing
    • C03B11/12—Cooling, heating, or insulating the plunger, the mould, or the glass-pressing machine; cooling or heating of the glass in the mould
    • C03B11/125—Cooling

Definitions

  • the present invention relates to a glass substrate for magnetic disk having a pair of principal surfaces and a manufacturing method thereof.
  • a hard disk device is incorporated in a personal computer, a notebook personal computer, and a DVD (Digital Versatile Disc) recording apparatus in order to record data.
  • a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and magnetic recording information is recorded in or read from a magnetic layer using a magnetic head (DFH (Dynamic Flying Height) head) that is slightly floated on a surface of the magnetic disk surface.
  • a glass substrate is suitably used as the substrate for the magnetic disk because substrate is hardly plastically deformed compared with a metallic substrate.
  • the magnetic recording density is being increased in order to correspond to a demand for an increase of a storage capacity in the hard disk device.
  • a magnetic recording information area is finely formed using a perpendicular magnetic recording system in which a magnetization direction of the magnetic layer is oriented toward a direction perpendicular to the substrate surface, which allows the storage capacity to be increased in one disk substrate.
  • a floating distance of the magnetic head from the magnetic recording surface is extremely shortened to form the fine magnetic recording information area.
  • the magnetic layer is formed flat such that the magnetization direction of the magnetic layer is oriented toward the direction substantially perpendicular to the substrate surface. Therefore, the glass substrate is formed such that surface irregularity of the glass substrate is decreased as much as possible.
  • the shortened floating distance of the magnetic head easily causes a head crush trouble or a thermal asperity trouble. Because these troubles are generated by the micro irregularity or a particle on the magnetic disk surface, the glass substrate is formed such that the surface irregularity in an end face is also decreased as much as possible in addition to the principal surface.
  • a press forming method and a floating method are well known as a method for manufacturing the sheet glass material that becomes a base of the glass substrate used in the magnetic disk.
  • Japanese Patent No. 3709033 discloses a press forming method as the method for manufacturing the glass material that becomes the base of the glass substrate used in the magnetic disk.
  • a glass gob made of molten glass is supplied onto a lower die that is a backing gob forming die, and press forming is performed to the glass gob using the lower die and an upper die that is a counter gob forming die.
  • the glass substrate used in the magnetic disk is manufactured by the following method: a glass gob made of molten glass is supplied onto a lower die that is a backing gob forming die; press forming is performed to the glass gob to prepare a sheet glass material using the lower die and an upper die that is a counter gob forming die; and the sheet glass material is formed into am information recording medium glass substrate.
  • the surface irregularity of the sheet glass material formed in accordance with the conventional method is not sufficient for the surface irregularity accuracy of the principle surfaces for the high density of the magnetic recording and the fine magnetic recording information area.
  • a mold release agent is applied to the die surface in order to prevent the glass material from fusing to the die surfaces of the upper die and lower die.
  • the surface roughness of the principal surface of the sheet glass material is increased because of the mold release agent.
  • the sheet glass material is easy to warp, and therefore good flatness of the formed sheet glass material is not achieved.
  • a machining allowance (ground quantity) is increased in the grinding process because the flatness is not so good with the conventional press forming method.
  • the machining allowance is about 200 ⁇ m with the conventional press forming method.
  • the machining allowance is increased in the grinding process, a deep crack is generated in the surface of the sheet glass material. Therefore, in a polishing process subsequent to the grinding process, the machining allowance (polishing quantity) is inevitably increased such that the deep crack is not left.
  • the machining allowance is increased in the polishing process in which the loose abrasive grain and the resin polisher are used, the neighborhood in the outer circumferential edge portion is rounded in the principal surface of the sheet glass material to cause a “roll-off problem” of the edge portion.
  • the sheet glass material having the sufficient flatness is not obtained with the conventional press forming method, it takes a relatively longer time to perform the grinding process that is the post-process, and then the “roll-off problem” is generated by the grinding process.
  • the sheet glass material is obtained by continuously flowing the molten glass in a bath filled with molten metal such as tin.
  • the molten glass is flown along a traveling direction in the bath to which an exact temperature operation is performed, and the belt-shaped glass ribbon is formed while finally adjusted to desired thickness and width.
  • the sheet glass material that becomes the base of the glass substrate used in the magnetic disk is cut out from the glass ribbon. Because the tin surface in the bath is kept horizontal, the sheet glass material obtained by the floating method has the sufficiently high surface flatness.
  • a predetermined impact resistance is required for the glass substrate used in the magnetic disk. Therefore, a chemically strengthening process is performed to the sheet glass material that becomes the base of the glass substrate in order to improve the impact resistance of the glass substrate.
  • the chemically strengthening process is performed as follows.
  • a mixed solution of potassium nitrate and sodium sulfate is used as a chemically strengthening solution.
  • the chemically strengthening solution is heated to 300° C. to 400° C.
  • the sheet glass material is dipped in the chemically strengthening solution for 3 to 4 hours. Therefore, ion replacement with sodium ion and potassium ion occurs in the surface layer of the glass material to form a compressive stress layer. Accordingly, the crack that is possibly generated in the surface of the glass material hardly progresses to the inside of the glass material.
  • the compressive stress layer formed through the chemically strengthening process has the thickness of about 50 to 200 ⁇ m.
  • the grinding process is required because the sheet glass material having the sufficiently flatness is not obtained.
  • the grinding process is required to remove the tin diffusion layer of the material surface.
  • an object of the invention is to provide a method for efficiently manufacturing the glass substrate for magnetic disk having the good surface irregularity accuracy and impact resistance.
  • a method for manufacturing a glass substrate for magnetic disk including a pair of principal surfaces including the steps of: performing press forming to molten glass to prepare a sheet glass material, the sheet glass material having a roughness of the principal surface of 0.01 ⁇ m or less and target flatness of a glass substrate for magnetic disk; chemically strengthening the sheet glass material by dipping the sheet glass material in a chemically strengthening salt containing an alkali metal ion to form a compressive stress layer at least on the principal surfaces of the sheet glass material, thereby preparing a disk substrate; polishing the principal surfaces of the disk substrate by pressing the polishing pad against the principal surfaces of the disk substrate while supplying a polishing solution including a polishing material between the disk substrate and the polishing pad, and relatively moving the disk substrate and the polishing pad.
  • a thickness of the sheet glass material prepared in the press forming step is larger than a target thickness of the glass substrate for magnetic disk by a polishing quantity of the principal surface polishing
  • the formed sheet glass material has the principal surface roughness of 0.01 or less and the target flatness of the glass substrate for magnetic disk.
  • the thickness of the sheet glass material is larger than the target thickness of the glass substrate for magnetic disk by the polishing quantity of the glass substrate for magnetic disk. Because the sheet glass material obtained through the press forming step has the good surface irregularity accuracy, only the polishing process is performed to the sheet glass material in the surface treatment process, but the grinding process having the machining allowance larger than that of the polishing process is not performed to the principal surface.
  • the good impact resistance is obtained by the compressive stress layer formed through the chemically strengthening process. Therefore, the glass substrate for magnetic disk having the good surface irregularity accuracy and impact resistance can efficiently be produced.
  • the press forming step includes the steps of: causing a lump of the molten glass to fall down; and forming the sheet glass material by performing the press forming to the lump while sandwiching the lump between surfaces of a pair of dies from both sides of a falling path of the lump, the dies being disposed opposite each other, the dies being set to substantially same temperature.
  • the sheet glass material having the principal surface roughness of 0.01 ⁇ m or less, the flatness necessary as the glass substrate for magnetic disk, and the target thickness of the glass substrate for magnetic disk can be prepared by the press forming.
  • the pair of dies is opened immediately after the press forming is performed to the lump while the lump is sandwiched between the surfaces of the pair of dies in the step of forming the sheet glass material.
  • the target flatness as the glass substrate for magnetic disk is 4 ⁇ m or less.
  • the manufacturing method of glass substrate for magnetic disk further comprises the step of scribing the sheet glass material between the forming step and the polishing step.
  • a glass substrate for magnetic disk that is manufactured by the above manufacturing method of glass substrate for magnetic disk has the principal surface with the flatness of 4 ⁇ m or less and the roughness of 0.2 nm or less.
  • FIG. 1A to FIG. 1C are views illustrating a magnetic disk prepared using a glass substrate for magnetic disk according to an embodiment of the invention
  • FIG. 2A to FIG. 2D are views illustrating a surface irregularity in a sheet glass material or a glass substrate
  • FIG. 3 is a view illustrating a flow of a manufacturing method for glass substrate for magnetic disk according to an embodiment of the invention
  • FIG. 4 is a plan view of an apparatus used in press forming of FIG. 3 ;
  • FIG. 5A to FIG. 5C are views illustrating an example of the press forming performed by the apparatus of FIG. 4 ;
  • FIG. 6A to FIG. 6C are views illustrating another example of the press forming performed by the apparatus of FIG. 4 ;
  • FIG. 7A to FIG. 7D are views illustrating still another example of the press forming performed by the apparatus of FIG. 4 ;
  • FIG. 8A to FIG. 8C are views illustrating still another example of the press forming performed by the apparatus of FIG. 4 ;
  • FIG. 9A and FIG. 9B are views illustrating a double-side polishing apparatus used in first polishing and second polishing of FIG. 3 .
  • FIG. 1A to FIG. 1C are views illustrating a magnetic disk that is prepared using a glass substrate for magnetic disk of the embodiment of the invention.
  • layers 3 A and 3 B including at least magnetic layers are formed on principal surfaces of a ring glass substrate 2 as illustrated in FIG. 1B .
  • an adhesive layer, a soft magnetic layer, a non-magnetic underlying layer, the perpendicular magnetic recording layer, a protective layer, and a lubricant layer are sequentially stacked.
  • a Cr alloy is used in the adhesive layer, and the adhesive layer acts as a bonding layer to the glass substrate 2 .
  • a CoTaZr alloy is used as the soft magnetic layer
  • a granular non-magnetic layer is used as the non-magnetic underlying layer
  • a granular magnetic layer is used as the perpendicular magnetic recording layer.
  • a material containing carbon hydride is used as the protective layer
  • a fluorine resin is used as the lubricant layer.
  • the magnetic disk 1 will be described with a more specific example.
  • a CrTi adhesive layer, a CoTaZr/Ru/CoTaZr soft magnetic layer, a CoCrSiO 2 granular non-magnetic underlying layer, a CoCrPt—SiO 2 .TiO 2 granular magnetic layer, and a carbon hydride protective layer are sequentially deposited in both the principal surfaces of the glass substrate 2 with an in-line type sputtering apparatus.
  • a perfluoropolyether lubricant layer is deposited on the uppermost layer by a dipping method to form the magnetic layers 3 A and 3 B.
  • magnetic heads 4 A and 4 B of a hard disk device float from surfaces of the magnetic disk 1 by 5 nm in the condition of high-speed rotation, for example, 7200 rpm, of the magnetic disk 1 . That is, a distance H in FIG. 1C is 5 nm.
  • the magnetic heads 4 A and 4 B record and read pieces of information in and from the magnetic layers.
  • the floating of the magnetic heads 4 A and 4 B can closely record and read the information in and from the magnetic layer of the magnetic disk 1 without sliding the magnetic heads 4 A and 4 B onto the magnetic layer, thereby realizing a fine magnetic recording information area and high density of the magnetic recording.
  • a central portion of the glass substrate 2 of the magnetic disk 1 to an outer circumferential edge portion 5 are precisely processed with target surface irregularity accuracy, and the magnetic heads 4 A and 4 B can be precisely operated while the distance H of about 5 nm is maintained.
  • a surface treatment process performed to the surface irregularity of the glass substrate 2 does not include a grinding process having a relatively large machining allowance, but only includes first polishing and second polishing processes having relatively small machining allowances.
  • the principal surface of the glass substrate 2 used in the magnetic disk 1 has surface irregularity in which flatness is 4 ⁇ m or less and surface roughness is 0.2 nm or less.
  • the flatness of 4 ⁇ M or less is target flatness required for the glass substrate for magnetic disk as a final product.
  • the flatness can be measured with a flatness tester FT-900 manufactured by NIDEK CO., LTD.
  • the roughness of the principal surface is expressed by arithmetic average roughness Ra defined by JIS B0601:2001.
  • the roughness ranges from 0.006 ⁇ m to 200 ⁇ m, for example, the roughness is measured with a roughness measuring machine SV-3100 manufactured by Mitutoyo Corporation, and the roughness can be computed by a method defined by JIS B0633:2001.
  • the roughness is measured with a scanning probe microscope (atomic force microscope) manufactured by SIT Nano Technology Inc, and the roughness can be computed by a method defined by JIS R1683:2007.
  • the sheet glass material of pre-polishing process was measured with the roughness measuring machine, and the glass substrate of post-polishing was measured with the scanning probe microscope (atomic force microscope).
  • FIG. 2A to FIG. 2D are views illustrating the surface irregularity.
  • the surface irregularity can be classified into four irregularities according to a wavelength of the irregularity.
  • the surface irregularity is classified into heave having the largest wavelength (wavelength of about 0.6 ⁇ m to about 130 mm), waviness (wavelength of about 0.2 ⁇ m to about 1 mm), micro-waviness (wavelength of 0.1 ⁇ m to 1 mm), and roughness (wavelength of 10 nm or less).
  • the heave can be expressed by the flatness as an index, and the roughness can be expressed by the arithmetic average roughness Ra as an index.
  • the sheet glass material of pre-polishing process obtained after the press forming has the following property: the roughness of the principal surface is 0.01 ⁇ m or less, and the flatness has the target flatness of the glass substrate for magnetic disk.
  • the sheet glass material prepared by the press forming has a thickness larger than the target thickness of the glass substrate for magnetic disk by the polishing quantity of the polishing process. That is, the thickness of the sheet glass material of the post-press forming is the thickness added to the target thickness of the glass substrate for magnetic disk of the final product by the small machining allowance of the polishing process.
  • the glass substrate for magnetic disk is prepared without performing the grinding process for adjusting the flatness and the thickness to the sheet glass material of the post-press forming.
  • the target flatness of the glass substrate for magnetic disk is 4 ⁇ m or less.
  • the reason the surface flatness of the sheet glass material is set to 4 ⁇ m or less is that the flatness of the glass substrate 2 used in the magnetic disk 1 is maintained and therefore the magnetic heads 4 A and 4 B can properly perform the recording and reading operations.
  • the sheet glass material having the principal surface roughness of 0.01 ⁇ m or less, the target flatness of the glass substrate for magnetic disk, and the thickness larger than the target thickness of the glass substrate for magnetic disk by the polishing quantity of the principal surface polishing can be achieved by the press forming of the embodiment.
  • the sheet glass material having the principal surface roughness of 0.01 ⁇ m or less, the target flatness of the glass substrate for magnetic disk, and the thickness larger than the target thickness of the glass substrate for magnetic disk by the polishing quantity of the principal surface polishing, cannot be achieved by the conventional press forming.
  • the glass substrate for magnetic disk having the flatness of 4 ⁇ m or less and the surface roughness of 0.2 nm or less can be obtained through the first polishing process and the second polishing process.
  • the reason the surface roughness of the sheet glass material of the post-press forming is set to 0.01 ⁇ m or less is because surface roughness can be adjusted to 0.2 nm by the two-time polishing process without increasing the machining allowance. Further, when the surface roughness is 0.01 ⁇ m or less, the scribing can efficiently be performed to the sheet glass material.
  • the surface irregularity of a sheet glass material G can be achieved by adjusting the surface roughness of the die in the press forming.
  • aluminosilicate glass, soda-lime glass, and borosilicate glass can be used as a material for the glass substrate 2 of the magnetic disk 1 .
  • the aluminosilicate glass can be suitably used in that chemically strengthening can be performed and in that the glass substrate for magnetic disk excellent for the flatness of the principal surface and the strength of the substrate can be prepared.
  • a chemically strengthened glass material mainly containing by molar percent of 57 to 74% SiO 2 , 0 to 2.8% ZnO 2 , 3 to 15% Al 2 O 3 , 7 to 16% LiO 2 , and 4 to 14% Na 2 O is suitably used as the aluminosilicate glass.
  • FIG. 3 is a view illustrating a flow of a manufacturing method for glass substrate for magnetic disk of the embodiment.
  • a first polishing process in Step S 50 and a second polishing process in Step S 70 constitute a surface treatment process of performing mirror-surface finishing to the principal surface of the sheet glass material.
  • the sheet glass material is prepared by the press forming (Step S 10 ).
  • the prepared sheet glass material has the principal surface roughness of 0.01 ⁇ m or less, the target flatness of the glass substrate for magnetic disk, and a thickness larger than the target thickness of the glass substrate for magnetic disk by a polishing quantity of the principal surface polishing.
  • the press forming is performed with an apparatus illustrated in FIG. 4 and FIG. 5 .
  • the press forming can also be performed with an apparatus illustrated in FIG. 6 , FIG. 7 , and FIG. 8 .
  • FIG. 4 is a plan view of an apparatus 101 used in the press forming
  • FIGS. 5 to 8 are views illustrating a state in which the apparatus performs the press forming when viewed from a side face.
  • An apparatus 101 illustrated in FIG. 4 includes four sets of press units 120 , 130 , 140 , and 150 and a cutting unit 160 .
  • the cutting unit 160 is provided on a path of the molten glass that flows out from a molten glass outflow port 111 .
  • a lump of the molten glass cut by the cutting unit 160 is caused to fall down, and the lump is pressed from both sides of the falling path of the lump while sandwiched between surfaces of a pair of dies set to the substantially same temperature, thereby forming the sheet glass material.
  • the four sets of press units 120 , 130 , 140 , and 150 are provided at intervals of 90 degrees around the molten glass outflow port 111 .
  • the substantially same temperature means that an absolute value of a temperature difference between a temperature at a press forming surface of first press forming die constituting the pair of dies and a temperature at a press forming surface of a second press forming die is 10° C. or less. More preferably the absolute value of the temperature difference is 5° C. or less, most preferably the absolute value of the temperature difference is 0° C.
  • the temperature at the press forming surface means a temperature near a center portion of the press forming surface.
  • the lump is brought into contact with the surfaces of the pair of dies in substantially the same timing, and the press forming is performed while the lump is sandwiched between the surfaces of the pair of dies in the substantially same timing.
  • the lump is brought into contact with the surfaces of the pair of dies in substantially the same timing means that the absolute value of the time difference between the time the molten glass lump comes into contact with one of the press forming surfaces and the time the molten glass lump comes into contact with the other press forming surface is 0.1 second or less. More preferably the absolute value of the time difference 0.05 second or less, most preferably the absolute value of the time difference is 0 second.
  • Each of the press units 120 , 130 , 140 , and 150 is driven by a moving mechanism (not illustrated) so as to be able to proceed and retreat with respect to the molten glass outflow port 111 . That is, each of the press units 120 , 130 , 140 , and 150 can be moved between a catch position and a retreat position.
  • the catch position position in which the press unit 140 is drawn by a solid line in FIG. 4
  • the retreat position positions in which the press units 120 , 130 , and 150 are drawn by solid lines in FIG. 4 and a position in which the press units 140 is drawn by a broken line in FIG. 4 ) is located away from the molten glass outflow port 111 .
  • the cutting unit 160 is provided on a path of the molten glass between the catch position and the molten glass outflow port 111 .
  • the cutting unit 160 forms the lump (hereinafter also referred to as “gob”) of the molten glass by cutting a proper quantity of the molten glass flowing out from the molten glass outflow port 111 .
  • the cutting unit 160 includes a pair of cutting blades 161 and 162 .
  • the cutting blades 161 and 162 are driven so as to intersect each other on the path of the molten glass at constant timing. When the cutting blades 161 and 162 intersect each other, the molten glass is cut to obtain the gob. The obtained gob falls down toward the catch position.
  • the press unit 120 includes a first die 121 , a second die 122 , a first driving unit 123 , and a second driving unit 124 .
  • Each of the first die 121 and the second die 122 is a plate-shaped member including a surface used to perform the press forming for the gob.
  • the first die 121 and the second die 122 are disposed such that normal directions of the surfaces become substantially horizontal, and such that the surfaces become parallel to each other.
  • the first driving unit 123 causes the first die 121 to proceed and retreat with respect to the second die 122 .
  • the second driving unit 124 causes the second die 122 to proceed and retreat with respect to the first die 121 .
  • Each of the first driving unit 123 and the second driving unit 124 includes a mechanism for causing the surface of the first driving unit 123 and the surface of the second driving unit 124 to be rapidly brought close to each other, for example, a mechanism in which an air cylinder or a solenoid and a coil spring are combined.
  • a first conveyer 171 , a second conveyer 172 , a third conveyer 173 , and a fourth conveyer 174 are provided below the retreat positions of the press units 120 , 130 , 140 , and 150 , respectively.
  • Each of the first to fourth conveyers 171 to 174 receive the sheet glass material G falling down from the corresponding press unit, and the conveyer conveys the sheet glass material G to an apparatus (not illustrated) of the next process.
  • the apparatus 101 is configured such that the press units 120 , 130 , 140 , and 150 sequentially move to the catch position and move to the retreat position while the gob is sandwiched, so that the sheet glass material G can continuously be formed without waiting for the cooling of the sheet glass material G in each press unit.
  • FIG. 5A to FIG. 5C more specifically illustrate the press forming performed by the apparatus 101 .
  • FIG. 5A is a view illustrating the state before the gob is made
  • FIG. 5B is a view illustrating the state in which the gob is made by the cutting unit 160
  • FIG. 5C is a view illustrating the state in which the sheet glass material G is formed by pressing the gob.
  • a molten glass material L G continuously flows out from the molten glass outflow port 111 .
  • the cutting unit 160 is driven in predetermined timing to cut the molten glass material L G using the cutting blades 161 and 162 ( FIG. 5B ). Therefore, the cut molten glass becomes a substantially spherical gob G G due to a surface tension thereof.
  • an outflow quantity per time of the molten glass material L G and a driving interval of the cutting unit 160 are adjusted such that a gob G G having a radius of about 10 mm is formed every time the cutting unit 160 is driven.
  • the made gob G G falls down toward a gap between the first die 121 and second die 122 of the press unit 120 .
  • the first driving unit 123 and the second driving unit 124 are driven such that the first die 121 and the second die 122 come close to each other at the timing the gob G G enters the gap between the first die 121 and the second die 122 . Therefore, as illustrated in FIG. 5C , the gob G G is captured (caught) between the first die 121 and the second die 122 .
  • An inner circumferential surface 121 a of the first die 121 and an inner circumferential surface 122 a of the second die 122 come close to each other with a micro gap, and the gob G G sandwiched between the inner circumferential surface 121 a of the first die 121 and the inner circumferential surface 122 a of the second die 122 is formed into a thin-plate shape.
  • a projected spacer 122 b is provided in the inner circumferential surface 122 a of the second die 122 in order to keep the gap between the inner circumferential surface 121 a of the first die 121 and the inner circumferential surface 122 a of the second die 122 constant.
  • a gap between the inner circumferential surface 121 a of the first die 121 and the inner circumferential surface 122 a of the second die 122 are adjusted such that the sheet glass material having the thickness larger than the target thickness of the glass substrate for magnetic disk by the polishing quantity of the principal surface polishing can be prepared.
  • a temperature control mechanism (not illustrated) is provided in each of the first die 121 and second die 122 , and temperatures at the first die 121 and second die 122 is retained sufficiently lower than a glass transition temperature T G of the molten glass L G .
  • the gob G G is formed into the substantially disk shape by spreading along the inner circumferential surface 121 a of the first die 121 and the inner circumferential surface 122 a of the second die 122 within an extremely short time, and the gob G G is rapidly cooled and solidified in the form of amorphous glass, thereby preparing the disk-shaped sheet glass material G.
  • the sheet glass material G is a disk-shaped plate having a diameter of 75 to 80 mm and a thickness of about 1 mm.
  • the press unit 120 quickly moves to the retreat position, instead the press unit 130 moves to the catch position, and the press unit 130 performs the pressing to the gob G G .
  • the first die 121 and the second die 122 are kept closed until the sheet glass material G is sufficiently cooled (until the sheet glass material G becomes at least a temperature below a yield point). Then, the first driving unit 123 and the second driving unit 124 are driven to separate the first die 121 and the second die 122 , the sheet glass material G falls down from the press unit 120 , and the conveyer 171 located below the press unit 120 receives the sheet glass material G (see FIG. 4 ).
  • the first die 121 and the second die 122 are closed within a time as extremely short as 0.1 second (about 0.06 second), and the molten glass substantially simultaneously comes into contact with the whole of the inner circumferential surface 121 a of the first die 121 and the whole of the inner circumferential surface 122 a of the second die 122 . Therefore, the inner circumferential surface 121 a of the first die 121 and the inner circumferential surface 122 a of the second die 122 are not locally heated, and a deformation is hardly generated in the inner circumferential surface 121 a and the inner circumferential surface 122 a .
  • the molten glass is formed into the disk shape before the heat transfers from the molten glass to the first die 121 and the second die 122 , a temperature distribution of the formed molten glass becomes substantially even. Therefore, in cooling the molten glass, the shrinkage quantity of the glass material has the small distribution, and the large deformation is not generated in the sheet glass material G. Accordingly, the principal surface flatness of the prepared sheet glass material G is improved better than that of the sheet glass material prepared by the conventional press forming, and the principal surface flatness of the prepared sheet glass material G can be set to 4 ⁇ m or less.
  • the surface roughness of the inner circumferential surface 121 a and the surface roughness of the inner circumferential surface 122 a are adjusted such that the arithmetic average roughness Ra of the sheet glass material G is 0.01 ⁇ m or less.
  • the substantially spherical gob G G is formed by cutting the flowing-out molten glass L G using the cutting blades 161 and 162 .
  • the glass does not become the substantially spherical shape only by cutting the molten glass L G , and the gob is not formed.
  • a gob forming die is used to form the gob.
  • FIGS. 6A to 6C are views illustrating a modification of the embodiment of FIG. 5 .
  • the gob forming die is used in the modification.
  • FIG. 6A is a view illustrating the state before the gob is made
  • FIG. 6B is a view illustrating the state in which the gob G G is made by the cutting unit 160 and a gob forming die 180
  • FIG. 6C is a view illustrating the state in which the press forming is performed to the gob G G to make the sheet glass material G.
  • the path of the molten glass L G to the press unit 120 is closed by closing the blocks 181 and 182 , and the lump of the molten glass L G cut with the cutting unit 160 is received by a recess 180 C formed by the block 181 and 182 .
  • the molten glass L G that becomes the spherical shape in the recess 180 C falls down toward the press unit 120 at one time by opening the blocks 181 and 182 .
  • the gob G G becomes the spherical shape by the surface tension of the molten glass L G .
  • the spherical gob G G is sandwiched between the first die 121 and the second die 122 to perform the press forming, thereby preparing the disk-shaped sheet glass material G.
  • FIGS. 7A to 7D in the apparatus 101 , instead of using the cutting unit 160 illustrated in FIGS. 6A to 6C , a moving mechanism that moves the gob forming die 180 in an upstream direction or a downstream direction along the path of the molten glass L G may be used.
  • FIGS. 7A and 7B are views illustrating the state before the gob G G is made
  • FIG. 7C is a view illustrating the state in which the gob G G is made by the gob forming die 180
  • FIG. 7D is a view illustrating the state in which the press forming is performed to the gob G G to make the sheet glass material G.
  • the recess 180 C formed by the blocks 181 and 182 receives the molten glass L G flowing out from the molten glass outflow port 111 .
  • the blocks 181 and 182 are quickly moved onto the downstream side of the flow of the molten glass L G at predetermined timing, thereby cutting the molten glass L G .
  • the blocks 181 and 182 are separated at predetermined timing. Therefore, the molten glass L G retained by the blocks 181 and 182 falls down at one time, and the gob G G becomes the spherical shape by the surface tension of the molten glass L G .
  • FIG. 7A the recess 180 C formed by the blocks 181 and 182 receives the molten glass L G flowing out from the molten glass outflow port 111 .
  • the blocks 181 and 182 are quickly moved onto the downstream side of the flow of the molten glass L G at predetermined timing, thereby cutting the molten glass L G .
  • the blocks 181 and 182 are separated at predetermined
  • the spherical gob G G is sandwiched between the first die 121 and the second die 122 to perform the press forming, thereby preparing the disk-shaped sheet glass material G.
  • FIGS. 8A to 8C are views illustrating another modification in which, instead of the gob G G , a lump C P of the optical glass heated by a softening furnace (not illustrated) is caused to fall down and the press forming is performed to the lump C P while the lump C P is sandwiched from both sides between dies 221 and 222 during the fall of the lump C P .
  • FIG. 8A is a view illustrating the state before the lump of the heated optical glass is formed
  • FIG. 8B is a view illustrating the state in which the lump of the optical glass falls down
  • FIG. 8C is a view illustrating the state in which the press forming is performed to the lump of the optical glass to make the sheet glass material G.
  • a glass material grasping mechanism 212 conveys the lump C P of the optical glass to a position above a press unit 220 .
  • the glass material grasping mechanism 212 releases the lump C P of the optical glass to cause the lump C P of the optical glass to fall down.
  • the lump C P is sandwiched between the first die 221 and the second die 222 to perform the press forming. Because the first die 221 and the second die 222 have the same configuration and action as those of the first die 121 and second die 122 illustrated in FIG. 5 , the descriptions are omitted.
  • Step S 20 After the press forming, scribing is performed to the formed sheet glass material G as illustrated in FIG. 3 (Step S 20 ).
  • the scribing means that two concentric (inside concentric and outside concentric) cutting-plane lines (scratch in the form of a line) are provided in the surface of the sheet glass material G with a scriber made of a super alloy or diamond particles in order to obtain the donut-shape (ring-shape) of the formed sheet glass material G having a predetermined size.
  • the sheet glass material G scribed into two-concentric-circle shape is partially heated, and a portion outside the outside concentric circle and a portion inside the inside concentric circle are removed by a difference in thermal expansion of the sheet glass material G, thereby obtaining the donut-shaped sheet glass material.
  • the cutting-plane line can suitably be provided with the scriber, because the sheet glass material G produced through the (a) press forming process has the roughness of 0.01 ⁇ m or less.
  • the scriber does not precisely trace on the surface, and cutting-plane line may not be evenly provided.
  • the sheet glass material may be prepared so as to have an outer diameter and circularity to an extent in which the scribing is not required, and a round hole is made in the sheet glass material with a core drill, thereby obtaining the ring sheet glass material.
  • the shape processing includes chamfering (chamfering of outer circumferential end portion and inner circumferential end portion). In the chamfering process, the outer circumferential end portion and inner circumferential end portion of the disk-shaped sheet glass material G are chamfered using a diamond abrasive grain.
  • the mirror-surface finishing is performed to an inner-circumferential-side end face and an outer-circumferential-side end face of the sheet glass material G by brush polishing.
  • slurry that includes fine particles such as cerium oxide as the loose abrasive grain is used.
  • the contamination of dust and damage such as a flaw are removed by performing the edge polishing. Therefore, generation of ions such as a sodium and potassium which cause corrosion can be prevented.
  • the first polishing is performed to the ground principal surface of the sheet glass material G (Step S 50 ).
  • the first polishing is intended to remove the flaw left on the principal surface and the deformation.
  • the first polishing has the machining allowance of several micrometers to about 10 micrometers.
  • the grinding process having the large machining allowance is not performed, the flaw and deformation that may be caused by the grinding process are not generated for the sheet glass material G. Therefore, the machining allowance can be reduced in the first polishing process.
  • a double-side polishing apparatus 3 illustrated in FIG. 9 is used in the first polishing process and the subsequent second polishing process.
  • the polishing is performed using a polishing pad 10 by relatively moving the sheet glass material G and the polishing pad 10 .
  • FIG. 9A is an explanatory view of a driving mechanism of the double-side polishing apparatus
  • FIG. 9B is a sectional view of a main part of the double-side polishing apparatus including upper and lower surface plates.
  • the double-side polishing apparatus 3 includes a polishing carrier attaching unit, an upper surface plate 31 , and a lower surface plate 32 .
  • the polishing carrier attaching unit includes internal gear 34 and a sun gear 35 , which are rotated at a predetermined rotation ratio.
  • the upper surface plate 31 and the lower surface plate 32 are reversely rotated while sandwiching the polishing carrier attaching unit therebetween.
  • the polishing pads 10 adhere to the surfaces of the upper surface plate 31 and the lower surface plate 32 that are facing each other.
  • a polishing carrier 33 is attached so as to engage the internal gear 34 and the sun gear 35 , and the polishing carrier 33 performs planet gear movement in which the polishing carrier 33 rotates while revolving around the sun gear 35 .
  • the plural sheet glass materials G are retained in the polishing carrier 33 .
  • the upper surface plate 31 can vertically be moved, and the upper surface plate 31 presses the polishing pad 10 against the pair of principal surfaces of the sheet glass material G as illustrated in FIG. 9B . While slurry (polishing solution) including the polishing abrasive grain (polishing material) is supplied, the sheet glass material G and the polishing pad 10 are relatively moved by the planet gear movement of the polishing carrier 33 and the mutual reverse rotations of the upper surface plate 31 and lower surface plate 32 , thereby polishing the pair of principal surfaces of the sheet glass material G.
  • a hard resin polisher is used as the polishing pad, and a cerium oxide abrasive grain is used as the polishing material.
  • the sheet glass material G is chemically strengthened (Step S 60 ).
  • a mixed solution of potassium nitride (60%) and sodium sulfate (40%) can be used as a chemically strengthening solution.
  • the chemically strengthening solution is heated to 300° C. to 400° C.
  • the washed sheet glass material G is pre-heated to 200° C. to 300° C.
  • the sheet glass material G is dipped in the chemically strengthening solution for three to four hours.
  • the dipping is performed while the plural sheet glass materials G are accommodated in a holder by retaining the sheet glass materials G at the end faces.
  • the sheet glass material G When the sheet glass material G is dipped in the chemically strengthening solution, the lithium ion and the sodium ion in the surface layer of the sheet glass material G are replaced with the sodium ion and the potassium ion which have relatively large ion radiuses in the chemically strengthening solution, respectively, thereby forming a compressive stress layer having a thickness of about 50 to 200 ⁇ m. Therefore, the sheet glass material G is strengthened to have good impact resistance.
  • the sheet glass material G to which the chemically strengthening treatment is performed is washed. For example, after washing the sheet glass material G using the sulfuric acid, the sheet glass material G is washed using pure water and IPA (isopropyl alcohol).
  • the second polishing is performed to the sheet glass material G to which the chemically strengthening treatment and washing are sufficiently performed (Step S 80 ).
  • the second polishing has the machining allowance of about 1 ⁇ m.
  • the second polishing is intended at the mirror-surface polishing of the principal surface.
  • the polishing is performed to the sheet glass material G using the double-side polishing apparatus 3 (see FIG. 9 ).
  • the second polishing process differs from the first polishing process in the polishing abrasive grain included in the polishing solution (slurry) used and a composition of the polishing pad 10 .
  • the particle diameter of the polishing abrasive grain used is decreased and hardness of the polishing pad 10 is softened.
  • a soft foaming resin polisher is used as the polishing pad, and a cerium oxide abrasive grain that is finer than the cerium oxide abrasive grain used in the first polishing process is used as the polishing material.
  • the sheet glass material G polished through the second polishing process is washed again.
  • the washing is performed using neutral detergent, pure water, and IPA.
  • the layers such as the magnetic layer are deposited on the glass substrate for magnetic disk 2 to prepare the magnetic disk 1 .
  • the sheet glass material having the surface irregularity in which principal surface flatness is 4 ⁇ m or less and the principal surface roughness is 0.01 ⁇ m or less can be formed by the press forming process. Therefore, the grinding process for improving the flatness and the grinding process having the machining allowance as large as about 200 ⁇ m are omitted after the press forming.
  • the manufacturing method of the embodiment because the sheet glass material is not produced by the floating method, the tin diffusion layer is not formed in the principal surface, and a grinding process is not therefore required to remove the tin diffusion layer.
  • the sheet glass material G has the machining allowance as small as about 10 ⁇ m, and the machining allowance is smaller than the thickness (about 50 to 200 ⁇ m) of the compressive stress layer. Therefore, there is not generated the conventional “roll-off problem” that is caused by the large machining allowance in the grinding process and therefore the large machining allowance in the polishing process.
  • the good impact resistance is obtained by the compressive stress layer that is formed through the chemically strengthening process after the press forming.
  • the imbalance of the compressive stress layer caused due to the tin diffusion layer formed only on one side of the principal surfaces does not occur because the sheet glass material is not produced by the floating method.
  • the glass substrate for magnetic disk having the good surface irregularity accuracy and impact resistance can efficiently be produced.
  • the chemically strengthening (Step S 60 ) is performed between the first polishing (Step S 50 ) and the second polishing (Step S 70 ).
  • the sequence is not limited to the embodiment.
  • the chemically strengthening (Step S 60 ) may appropriately be replaced.
  • the first polishing process, the second polishing process, and the chemically strengthening process (hereinafter, the process sequence 1 ) may be performed in this order.
  • the process sequence illustrated in FIG. 3 is performed more preferably.
  • the alminosilicate glass (57 to 74% SiO 2 , 0 to 2.8% ZnO 2 , 3 to 15% Al 2 O 3 , 7 to 16% LiO 2 , and 4 to 14% Na 2 O) was used as the glass material.
  • the magnetic layer was formed on the prepared glass substrate using in-line type sputtering apparatus. Specifically, the CrTi adhesive layer, the CoTaZr/Ru/CoTaZr soft magnetic layer, the CoCrSiO 2 granular non-magnetic underlying layer, the CoCrPt—SiO 2 .TiO 2 granular magnetic layer, and the carbon hydride protective layer were sequentially deposited on both the principal surfaces of the glass substrate. Then the perfluoropolyether lubricant layer was deposited on the deposited uppermost layer by a dipping method, thereby obtaining the magnetic disk.
  • the glass substrate was prepared through Steps S 20 to S 70 illustrated in FIG. 3 using the sheet glass material having the principal surface flatness of 4 ⁇ m or less and the principal surface roughness ranging from 0.001 ⁇ m to 0.01 ⁇ m.
  • the sheet glass material was formed by the press forming method illustrated in FIGS. 4 and 5 .
  • the glass substrate was prepared through Steps S 20 to S 70 illustrated in FIG. 3 using the sheet glass material having the principal surface flatness of 4 ⁇ m or less and the principal surface roughness ranging from 0.011 ⁇ m to 1.334 ⁇ m.
  • the sheet glass material was formed by the press forming method illustrated in FIGS. 4 and 5 .
  • the glass substrate was prepared through Steps S 20 to S 70 illustrated in FIG. 3 using the sheet glass material having the principal surface flatness of 4 ⁇ m or less and the principal surface roughness ranging from 0.001 ⁇ m to 0.002 ⁇ m.
  • the sheet glass material was formed by the floating method.
  • the glass substrate was prepared through Steps S 20 to S 70 illustrated in FIG. 3 using the sheet glass material having the principal surface flatness exceeding 4 ⁇ m and the principal surface roughness ranging from 0.004 ⁇ m to 0.006 ⁇ m.
  • the sheet glass material was formed by the conventional press method.
  • the first polishing process the polishing was performed using a cerium oxide abrasive grain (average particle size; diameter of 1 ⁇ m to 2 ⁇ m) as a polishing material and a hard urethane pad as a polishing pad, and the machining allowance of about 3 ⁇ m.
  • the second polishing process the polishing was performed using colloidal silica (average particle size; diameter of 0.1 ⁇ m) as a polishing material and a soft polyurethane pad as a polishing pad, and the machining allowance of about 1 ⁇ m.
  • the flatness and surface roughness (the flatness and surface roughness of post-forming) of the glass substrate obtained by each of Examples and Comparative examples were measured.
  • the LUL (Load/UnLoad) endurance test (600,000 times) was performed to evaluate floating stability of the magnetic head with respect to the magnetic disk that was prepared based on the glass substrate obtained by each of Examples and Comparative examples.
  • Table 1 illustrates the roughness of the principal surface, forming method, and flatness (pre-forming and post-forming) in Examples 1 to 5 and Comparative examples 1 to 11 and the LUL endurance test result (pass or fail).
  • “OK” indicates that the surface roughness satisfies a criteria of 0.2 nm or less (the criteria required as the glass substrate for magnetic disk), and “NOK” indicates that the surface roughness does not satisfy the criteria of 0.2 nm or less.
  • the glass material formed by the press forming method illustrated in FIGS. 4 and 5 has the flatness of 4 ⁇ m or less and the surface roughness of 0.01 ⁇ m or less (Examples 1 to 5), the surface roughness reaches the criteria (0.2 nm or less) only by the first polishing process and the second polishing process. In such cases, the glass material passes the LUL endurance test.
  • the glass material formed by the floating method has the good surface roughness and flatness
  • a warp is generated due to a stress difference of the surfaces, because there is a difference of ion exchange between one surface in which the tin diffusion layer exists and the other surface in which the tin diffusion layer does not exist by the chemically strengthening process, and the flatness degraded by the warp is not improved only by the first polishing process and the second polishing process.
  • the glass material fails in the LUL endurance test.
  • the glass material formed by the conventional press forming method has the flatness exceeding 4 ⁇ m and the surface roughness of 0.01 ⁇ m or less (Comparative examples 9 to 11), the flatness does not satisfy the criteria (4 ⁇ m or less) only by the first polishing process and the second polishing process. In such cases, the glass material fails in the LUL endurance test.

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